The present disclosure relates to a developing device for developing an electrostatic latent image using toner, and an image forming apparatus including the developing device.
There is an image forming apparatus such as an multifunction peripheral, a copier, a printer, or a facsimile, which develops with toner an electrostatic latent image formed on a photosensitive drum so as to perform printing. Further, there is an image forming apparatus which uses a developer containing magnetic carrier and toner (so-called two-component developer). Further, in the developing process using the two-component developer, it is not appropriate for a magnetic brush of the carrier to directly contact with the photosensitive drum from viewpoints of image quality and the like. Therefore, there is known an image forming apparatus including the developing device of a type including a developing roller opposed to the photosensitive drum so as to carry the toner, in which a magnetic roller opposed to the developing roller forms the magnetic brush so that only the toner is transferred to the developing roller by the magnetic brush. Thus, the magnetic brush does not contact with the photosensitive drum when developing the electrostatic latent image (This may be referred to as “touch down developing” or “hybrid developing”). This method is advantageous to a one-component developing method or a conventional two-component developing in various points such as image quality, printing speed, life of toner, carrier scatterproof, and the like.
In the touch down developing method as described above, alternating voltage (for example, approximately 1 to 2 kV in peak-to-peak voltage) is applied to the developing roller. In this way, the charged toner is scattered for developing the electrostatic latent image. Further, there is a case where a signal indicating ON/OFF of conduction (switching) is generated using a switching element such as a transistor, a DC component of the signal is removed using a capacitor, the signal is supplied to a primary of a transformer, and the alternating voltage to be applied to the developing roller is obtained from a secondary of the transformer.
Here, there is a case where a duty ratio of the switching should be changed from a viewpoint of preventing occurrence of leakage (discharge) between the photosensitive drum and the developing roller or preventing occurrence of an uneven toner image. However, when the duty ratio of the switching is changed, an imbalance voltage (having offset energy) is applied to the transformer. As a result, asymmetric magnetization may occur in the transformer. When the asymmetric magnetization occurs so that magnetic flux is biased, the transformer becomes in such a DC biased state. Then, current larger than rated current (overcurrent) may flow, and hence there is large probability that the switching element is broken down. In particular, as a transient variation of the duty ratio is larger, larger asymmetric magnetization occurs so that larger current is apt to flow in the switching element.
Further, when the duty ratio is once changed, voltage between electrodes of the capacitor is periodically fluctuated due to energy oscillation between the primary of the transformer and the capacitor. Further, in relation to this energy oscillation, there is a problem that when the duty ratio is changed during a period of the fluctuation of the voltage between electrodes of the capacitor, there is a timing (time slot) in which large current flows in the switching element.
Note that in the above-mentioned known image forming apparatus, a hybrid developing device is realized which is advantageous in points such as image quality, life of toner, carrier scatterproof, printing speed, and the like, without adding a new member or the like. However, the apparatus does not resolve the problem that there is possibility that large current may flow in the switching element when the duty ratio of switching is changed in the switching element.